In semiconductor processing, multiple lithography steps are generally used to form a semiconductor chip. These steps typically include forming a photoresist over a substrate, exposing the photoresist to a pattern of light generally controlled by a mask, developing the pattern in the photoresist to expose the underlying substrate, and etching the pattern in the underlying substrate. The pattern etched in the underlying substrate may be a basis for some feature formation, such as an ion impurity implantation such as for doping source and drain regions, a formation of a structure like a gate pattern, or a pattern for a conductive material such as in a metallization layer.
Photoresists may be classified into two groups: positive resists and negative resists. A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The portion of the photoresist that is unexposed remains insoluble to the photoresist developer. A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
Advances in semiconductor processing have generally allowed for continued reduction of minimum feature sizes for semiconductor chips, which have increased the requirement on the image resolution of the mask used in lithography. If a mask is not able to precisely form a pattern in a photoresist, the subsequently formed feature may not meet its critical dimension requirement.
For advanced lithography imaging, the improvement of resolution is one tough challenge. Conventional positive tone imaging (PTI) technology has reached its limit and cannot get any better image resolution with enough process window. The negative tone imaging (NTI) technology using a bright mask has the potential to provide a better resolution than the PTI technology using a dark mask. However, current NTI technology using a bright mask suffers serious substrate loss during the developing process. Therefore there is a need to develop a better bright mask used in the NTI technology for advanced lithography imaging.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.